1. Field of the Invention
The present invention generally relates to thermal imaging systems (sometimes also referred to as FLIRs) for detecting scene information and displaying an image of that information. More specifically, it relates to such systems which utilize a pyroelectric detector that is periodically reset.
2. Description of Related Art
It is known in the field of thermal imaging systems to display an image of a scene by receiving scene information through an optical system that focuses the scene information on a detector. In response to the scene information, the detector generates electrical signals representative of the scene. These are processed and used to create an image on a display device.
A pyroelectric uncooled focal plane array detector used in this type of system is shown in Shaham, U.S. Pat. No. 4,740,700. The uncooled pyroelectric detector elements in this system are AC coupled. The focal plane array must therefore be periodically reset or normalized. Otherwise, the scene information must change during operation.
To image a static or non-changing scene, the signals produced by the scene information usually must be stored and subtracted from the signals produced while the scene information is blocked from the detector. This is usually accomplished by a mechanical chopper positioned between the optical system and the detector in the system. The mechanical chopper alternately passes and blocks the scene information, thereby effectively resetting the detector elements of the focal plane array. The normal scene signal is then electronically subtracted from the blocked scene signal.
The use of a mechanical chopper complicates the system. It must be introduced in the optical path. This requires a moving mechanical part which reduces reliability and increases cost. A need therefore exists for a workable thermal imaging system which does not need a mechanical chopper.